Organic EL devices are know to be efficient and capable of producing a wide range of colors and are useful for many applications such as flat panel display.
Organic EL devices generally have a layered structure with an organic luminescent medium sandwiched between an anode and a cathode. The organic luminescent medium usually refers to an organic light emitting material or a mixture thereof in the form of a thin amorphous or polycrystalline film. Representatives of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322–334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. In these prior arts, the organic luminescent medium was formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terpheyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic luminescent medium was present as a single layer having a thickness much above 1 micrometer. The voltage required to drive the EL devices was as much as a few hundreds volts, thus the luminous efficiency of these EL devices was rather low.
In U.S. Pat. No. 4,356,429, Tang further advanced the art of organic EL device by disclosing a bi-layer EL device configuration. The organic luminescent medium in this bi-layer configuration comprises of two extremely thin layers of organic film (<1.0 micrometer in combined thickness) sandwiched between the anode and cathode. The layer adjacent to the anode, termed the hole-transport layer, is specifically chosen to transport predominantly holes only in the EL device. Likewise, the layer adjacent to the cathode is specifically chosen to transport predominantly electrons only in the EL device. The interface or junction between the hole-transport layer and the electron-transport layer is referred to as the electron-hole recombination zone where the electron and hole recombine to produce electroluminescence with the least interference from the electrodes. This recombination zone can be extended beyond the interface region to include portions of the hole-transport layer or the electron-transport layer or both. The extremely thin organic luminescent medium offers reduced electrical resistance, permitting higher current densities for a given voltage applied on the EL device. Since the EL intensity is directly proportional to the current density through the EL device, this thin bi-layer construction of the organic luminescent medium allows the EL device to be operated with a voltage as low as a few volts, in contrast to the earlier EL devices. Thus, the bi-layer organic EL device has achieved a high luminous efficiency in terms of EL output per electrical power input and is therefore useful for applications such as flat-panel displays and lighting.
For the production of full-color EL display panel, it is necessary to have efficient red, green, and blue (RGB) EL materials with proper chromaticity and sufficient luminance efficiency. A doped EL system based on the principle of guest-host energy transfer to effect the spectral shift from tris-(8-hydroxyquinolinato)aluminum (Alq) to the dopant molecules has been disclosed by Tang et al in U.S. Pat. No. 4,769,292. The guest-host doped system offers a ready avenue for achieving such an objective, mainly because a single host with optimized transport and luminescent properties may be used together with various guest dopants leading to EL of desirable hue. It usually can be achieved by applying the three layer organic EL device that contains a light-emitting layer between the hole transport layer and electron transport layer that has been disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610–3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. The host materials in light-emitting layer can be electron transport materials, such as 8-hydroxyquinoline aluminum complex [U.S. Pat. No. 4,769,292], the hole transport materials, such as aryl amines [Y. Hamada, T. Sano, K. Shibata and K. Kuroki, Jpn. J. Appl. Phys. 34, 824, 1995], or the charge injection auxiliary materials, such as stilbene derivatives [C. Hosokawa et al., Appl. Phys. Lett., 67(25) 3853, 1995]. The doped guest materials, also known as the dopant, are usually chosen from highly fluorescent dyes. In the three layer organic EL device, the light-emitting layer provides an efficient site for the recombination of the injected hole-electron pair followed by the energy transfer to the guest material and produces the highly efficient electroluminescence.
Alq is only suitable host for green and red EL emitters since its emission at 530 nm is adequate to sensitize guest EL emission in the green and red spectral region. In general, the host material in the light emitting layer should be as luminescent as possible and also the luminance wavelengths are desired to be in the blue or near the UV region. The latter attribute is important for down-shifting of the EL emission wavelength in a host-guest emitter layer that is able to produce blue, green, red, and white light output.
Shi et. al. in U.S. Pat. Nos. 5,935,721 and 5,972,247 has disclosed organic electroluminescent (EL) element, that belongs to 9,10-di-(2-naphthyl)anthracene and 9,10-bis(3′5′-diaryl)phenyl anthracene derivatives derivatives, provides a thermally stable, glassy, and highly fluorescent materials in condensed thin film which dramatically exhibits different EL performance than that of 9,10-(diphenyl)anthracene derivatives. As a result, organic EL device employing these anthracene derivatives in light-emitting layer produce a bright blue emission and long operational stability. In accordance with the present invention, Shi et. al also taught that thiese anthracene derivatives are extremely useful for the production of full color EL display panel. With these anthracene derivatives as host materials, an appropriate EL hues or colors, including white, have also been produced by a downhill energy transfer process. For example, a green EL emission has been produced by doping into the anthracene derivatives with small amount of a green fluorescent sensitizing dye. This host-guest energy transfer scheme has been discussed in detail by Tang et al. in U.S. Pat. No. 4,769,292. A white EL emission has been produced by combination of two emissions. This scheme has been disclosed in detail by Shi et. al, U.S. Pat. No. 5,683,823.
In order to achieve the best performance of light output through the guest-host doped system, especially through the light emitting layer that consists of host and dopant materials. The dopant materials play an important role in term of enhance the light output efficiency, color purity and device operational stability. There are only a few of classes materials have been successfully used to produce blue emission. One of these is stilbene derivatives containing arylamino-groups. [C. Hosokawa et al., Appl. Phys. Lett., 67(25) 3853, 1995]. However, the liable arylamino-groups is not preferred in achieving desired device stability. Another is perylene and its derivatives used by Kodak. the small molecular size of perylene is not preferred in fabrication process. Modifying the perylene derivatives to increase the molecular size usually limited by spectrum shift away from pure blue emission.